Editing Bipolar junction transistor (section)

=== Transistor characteristics: alpha (''α'') and beta (''β'') <span class="anchor" id="Alpha"></span><span class="anchor" id="Beta"></span><span class="anchor" id="AlphaBeta"></span> ===
The proportion of carriers able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. A thin and lightly doped base region means that most of the minority carriers that are injected into the base will diffuse to the collector and not recombine.

==== Common-emitter current gain ====
The ''[[common-emitter]] current gain'' is represented by {{mvar|β}}<sub>F</sub> or the [[Two-port network#h-parameters|{{mvar|h}}-parameter]] {{mvar|h}}<sub>FE</sub>; it is approximately the ratio of the collector's direct current to the base's direct current in forward-active region. (The F subscript is used to indicate the forward-active mode of operation.) It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications. Both injection efficiency and recombination in the base reduce the BJT gain.

==== Common-base current gain ====
Another useful characteristic is the ''[[common-base]] current gain'', {{mvar|α}}<sub>F</sub>. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.980 and 0.998. It is less than unity due to recombination of charge carriers as they cross the base region.

Alpha and beta are related by the following identities:
: <math>\begin{align}
  \alpha_\text{F} &= \frac{I_\text{C}}{I_\text{E}}, & \beta_\text{F} &= \frac{I_\text{C}}{I_\text{B}}, \\
  \alpha_\text{F} &= \frac{\beta_\text{F}}{1 + \beta_\text{F}} & \iff \beta_\text{F} &= \frac{\alpha_\text{F}}{1 - \alpha_\text{F}}.
\end{align}</math>

Beta is a convenient figure of merit to describe the performance of a bipolar transistor, but is not a fundamental physical property of the device. Bipolar transistors can be considered voltage-controlled devices (fundamentally the collector current is controlled by the base–emitter voltage; the base current could be considered a defect and is controlled by the characteristics of the base–emitter junction and recombination in the base). In many designs beta is assumed high enough so that base current has a negligible effect on the circuit. In some circuits (generally switching circuits), sufficient base current is supplied so that even the lowest beta value a particular device may have will still allow the required collector current to flow.